The present invention relates generally to non-aqueous lithium ion secondary electrochemical cells and batteries formed of such cells and, more particularly, to such cells and batteries in which an anode comprising a substrate formed of carbon based material provides the cells and batteries with overcharge and overdischarge acceptance ability.
Since its introduction and commercialization in 1991, lithium-ion battery systems have received considerable interest not only to the battery community but also to the to electronic industries. In lithium-ion batteries, carbon or graphite is used as an anode, a lithiated transition metal intercalation compound is used as a cathode and LiPF6 is used as an electrolyte in carbonate-based organic solvents. For example, the reactions at the electrodes and overall cell reaction of an oxide-containing lithium intercalation compound are as follows: 
where LiMO2 represents the lithiated metal oxide intercalation compound.
The electrochemical process is the uptake of lithium ions at the anode during charge and their release during discharge, rather than lithium plating and stripping as occurs in metallic lithium rechargeable battery systems. As metallic lithium is not present in the cell, lithium-ion cells have enhanced safety and a longer cycle life than the cells containing metallic lithium. Because of their advantageous characteristics, lithium-ion batteries are widely used for consumer electronics applications such as cellular phones, laptop computers, camcorders, and personal digital assistant.
At present, hard carbon or graphite is used as active anode material in commercial lithium-ion batteries. Polyvinyledene fluoride (PVDF) is used as a binder to improve the mechanical integrity of the electrode. Copper is universally used as the substrate for anode. Hard carbon or graphite material is mixed with PVDF in an organic solvent and the mixture is coated on the copper substrate to produce the anode.
The present state-of-the-art lithium-ion cells require overcharge/overdischarge protection circuits and/or devices so that the cells can charge and discharge within certain voltage regimes. Overdischarge causes dissolution of copper that is used as the substrate for carbon anode and degrades cell performance.
During overcharge, more lithium-ions are transported to the carbon anode and since the anode does not have enough room to accommodate them, overcharge may cause metallic lithium deposition on the anode, heat build-up, and ultimately thermal run away of the cell.
Overcharge/overdischarge protection circuits and/or devices also increase(s) the weight and cost of the lithium-ion cell. Reliable and inexpensive overcharge/overdischarge protection for multi-cell lithium-ion battery is a major obstacle to commercialization of the systems in electric vehicle and other high voltage applications.
Accordingly, it is the primary objective of the present invention to improve the overcharge/overdischarge acceptance ability of lithium-ion cells and battery systems.
Another objective of the present invention is to provide such overcharge/overdischarge acceptance ability in the anodes of the cells, and thereby eliminate the need for overcharge/overdischarge protection circuits and/or devices, thus reduce the cost and weight of lithium-ion cells.
Still another objective of the present invention is to provide a novel and improved rechargeable lithium-ion cell and battery system that utilizes carbon-based substrate to produce an anode which provides such overcharge/overdischarge acceptance ability.
Briefly stated, this invention provides a rechargeable electrochemical cell comprising a body of aprotic, non-aqueous electrolyte, first and second electrodes in effective contact with said electrolyte, the first electrode comprising a positive material such as lithiated intercalation compound and the second electrode comprising carbon on carbon-based substrate. In accordance with this invention, commercially available carbon based films of high electronic conductivity are chosen for the substrate, and on which carbon material having high lithium-ion intercalation capacity is coated to produce the negative electrode, i.e., anode of the electrochemical cell. Commercially available carbon-carbon composite of high electronic and thermal conductivity can also be chosen as the anode of the electrochemical cell. The substrate of the carbon-carbon composite anode is carbon and, therefore, the lithium-ion cells made in accordance with the present invention can accept repeated overdischarge without performance degradation.
The anode substrate and anode itself each comprises carbon material, and each can accept lithium-ions during charge from cathode of lithiated intercalation compounds. The substrate can, therefore, act as a sink of lithium-ions. During overcharge, additional lithium-ions from the cathode can, therefore, be stored to the anode substrate without causing metallic lithium deposition.
Thus, the anode made according to the present invention allows the acceptance ability for overcharge and overdischarge of a lithium-ion cell.